Electrostatic coating



Jan. 7, 1964 R. L. HlNEs ELECTROSTATIC COATING Filed Aug. 24, 1962 R. L.H/NES INVENTOR.

e By g PA /N 7' SOLVENT ATTORNEYS llnited States Patent O 3,137,029ELECTRGSTATEC CATENG Roderick L. Hines, North Eroe-lr, lll., assigner toFord Motor Company, Dearborn, Mich., a corporation of Delaware FiledAug. 24, 1962, Ser. No. 22l,397 6 Claims. (jl. tlg- 621) rl`hisinvention relates to the coating of articles by electrostaticdeposition. in particular, this invention relates to the application ofa fluid coating material to articles by eleotrostatically atomizing,projecting and depositing the coating upon tue articles within anelectrostatic field created between the Varticles and an issuing sourceof the coating material. More particularly, this invention relates to anapparatus for electrostatic deposition wherein the lluid to be sprayedis electrostatically atomized into discrete droplets from a curvedsurface.

Tais application is a continuation in part of my copending application,U5. Serial No. 574,325, filed March 27, i956, now aoandoned.

The broad concept of electrostatic atomizing, proiecg and depositing isknown to the art. The improvement in electrostatic coating provided bythis invention comprises atomization of the coating liquid from anelectrica ly charged, curved surface having both significan length andsignificant breadth under essentially corona ree conditions.

ln electrostatic coating processes, as generally practreed, the fluid tobe atomized is fed over a surface or through a small hole to a sharpedged electrode between which and the article a potential difference ofsufficient magnitude is maintained to create an electrostatic field witha corona dischange the region adjacent such edge. ln rnost, if not all,commercial processes, centrifugal forces are utilized in conjunctionwith electrostatic forces to provide tie desired atomization. lt wasconsidered necessary that the discharge electrode be provided with avery sharp radius of curvature in order to provide an electrical chargeconcentration suilicient to produce corona discharge. Apparatus andmethods representative of these point-emission or edge-emission systemsare disclosed in U3. Patents 2,722,908, 2,685,536 and 2,658,909.However, these methods have exhibited certain undesirablecharacteristics resulting from the `difficulties inherent in attemptingto control a corona discharge. These have led to a search for auxiliaryor secondary control systems to reduce or eliminate the resultingirregular scattering of coating material. The problem of 'fluctuationsin the intensity of the iield resulting from corona discharge from asharp edge is discussed in the above cited US. Patent 2,658,069.

lt now has been discovered that a highly effective electrostatic coatingprocess can be effected by atornization from a curved surfe-.ce having abroad, as contrasted to a sharp, angle of curvature under essentiallycorona free conditi ns. ln accordance with invention, the vatomizationis carried out `from a relatively smooth, curved surfe. e, the radius ofcurvature of which is above about one-fourth inch. 'he design of theapplicator head may be varied in accordance with the broad principles ofthe method of this invention including, but not by way of limitation,the spherical and cylindrical heads hereinafter discussed in greaterdetail.

The characteristic advantages of the herein'cefore and hereinafterdescribed invention are: (l) The process admits of greater versatilityin design of the applicator head, which (a) Permits coating ofrelatively large objects using an applicator head that is relativelycompact and eliminating a requirement ott edge atoinization that ICC theapplicator head be substantially co-extensive with the surface to becoated, and facilitates adaptation of system to the coating of a varietyof articles.

(2) Surface atomizat-ion admits of positioning a lmaximum number ofatomica-tion sites within a given radius about the center of theatomization area and hence maximizes atomization rate from such are-a.

(3) Surface atomization permits corona free operation and greatercontrol of droplet size at high -atomization ra es, thus providing,

(a) a more uniform coating of the article or work piece, and

(b) a substantial decrease in volume requirements for paint.

(4) Surface atomization by eliminating the narrow atomization site ofthe edge or point reduces limitations on the viscosity of the coatingliquid.

(5) Corona free surface atomization reduces electrical requirements perunit of coating material atomized and deposited with a resultingincrease in safety and a decrease in the complexity and quality of theelectrical system employed to produce atomization. rl`his invention willbe more fully understood from the accompanying drawings which illustratetwo representative embodiments of this invention.

ln tbe drawings:

FIGURE l is a schematic side view of one embodiment of the apparatusused in the process of this invention A.tf-@URE 2 is a partiallycut-away view, to a larger scale, of the applicator head of FlGURE l.

FGURE 3 is a partially cut-away view of the applicator head of anotherembodiment of this invention.

FIGURE 4 represents a greatly magnilied view in cross section of jets ofcoating liquid extending from the applicator head in accordance with oneembodiment of the invention.

Referring more particularly to TFlGURE 1, reference numeral l. indicatesgenerally an applicator, atomizer or spray head. Numeral 2 is a metalsupporting element having `a horizontal bore o. Support ele-ment 2 isconnected to a suitable (DC.) high voltage source i9 through insulatedcable 3 to maintain the atomizer head at a rather high potential withrespect to ground. Support element 2 is, in turn, supported by stand l7which is made of a suitable insulating material. Delivery tube d,passing through horizontal bore e, provides communication between theinterior of head l and supply tube S and is secured in position by setscrew l2. Tube d may be metal to provide an electrical connectionbetween atomizer head 'l and support element 2. ln the alternative,other means of electrical connection may be einployed to introduce therequisite charge to head l. Support element 2 is preferably spherical toeliminate sharp edges which tend to cause local buildup of charges uponthe element and lead to a corona discharge, thereby impaiiing theeiicacy olf the system by reducing the intensity of the field. Tube S`is connected by suit-able means to end ld `of tube 4 which protrudes`from support 2 and together with tube d and branch tube ld provides allow path from paint reservoir l5 and solvent reservoir lo to atomizerhead l. Tubes 5 and i4 are made of an insulating material, eg. rubber,polyethylene, polypropylene, etc., to prevent the charge on supportelement 2 and tube ifrom leaking to ground. Paint reservoir l5 fromwhich tube 5 is pressure-d by suitable means, not shown, in order toprovide the necessary pressure to ilov/ the paint to the atomizingsurface of the atomizer head. rl`his pressure need only be sti-.flicientto overcome the static head between the atomizing surface and thereservoir plus the pressure ldrop through holes Z2 and may be eliminatedentirely if the reservoir is located a sufcicnt distance above theatomi/;l g surface. Solvent reservoir le is also pressured for sii rreasons. Ordinarily,

lo the paint or other coating material is minted with a solvent and! orthinner before entering paint reservoir l5. However, the relativeproportions of paint and solvent can be controlled by manipulation ofmixing valves S and g in ines 5 and i4 respectively and the rate of owof paint and solvent to the atomizing head can be controlled bymanipulation of Avalve 7 in line 5. ln the event it is desired to purgethe system, the paint supply can be shut olf via valve S and a stream ofsolvent run through the lines. Numeral 18 refers to the article to becoated.

Referring now to FIGURE 2, it is seen that atomizer head is a hollowsphere having iiuid inlet 23 connected by suitable means such as weldingor soldering to tube 4. The hollow interior of the head forms an innerpaint reservoir 2l. The exterior surface 20 of head l constitutes theatomizing surface. A series of equally spaced holes 22 connect theatomizing surface 20 with paint reservoir 2l. In the embodiment shown,these holes cover one-half of the sphere but, depending on the nature ofthe object to be sprayed, these holes may cover more or less of thesphere.

Preferably, the diameter of holes Z2 will be only slightly larger thanthe diameter necessary to permit the paint or other coating material toflow without plugging. This, of course, depends upon the coating liquidthat is being sprayed; lbut it has been found that hole diameters in the.range of about 0.002 to 0.020, preferably above about 0.005, inch aresatisfactory for the common paints and lacquers. `The relationshipbetween the diameter of the hole and the length of the hole, i.e., the4thickness of the exterior wall of the atomizer head, can be adjusted toprovide the desired pressure drop. In most applications, a wallthickness of about 0.005 to 0.065 will be preferred for the atomizerhead. This, of course, determines the length of the holes. With moreviscous coating liquids, the upper lim-it of this range `can beextended. The spacing of holes 22 is hereinafter discusse-d in detailwith respect to different embodiments.

As aforementioned, the radius of curvature of the atomizing surfaceshould be above about one-fourth inch. While there is no sharp upperlimit to this measurement, the radius ed curvature Ifor most practicaloperating conditions Vwill be in the range of about 0.25 to 3.0,preferably about 0.40 to 1.25 inches.

FGURE 3 shows another embodiment of the applicator or atomizer headwherein coating material is atomized from a cylindrical surface. lnFIGURE 3 attention is particularly directed to outlet 24. This outletprovides means whereby the liquid not passing through the holes 2S toatomization surface 26 may be recycled back to the liuid supply line S,shown in FIGURE l, thereby permitting higher flow rates through thesystem if it is desired. This arrangement is useful for returningforeign particles or aggregates of particles in the coating liquid tothe supply sources and helps to prevent plugging of the holes of theatomizer head. If it is desired, a lilter may be inserted in a recycleline to remove objectionable oversize particles.

Referring again to FGURE l, the atomizer head 1 is, in operation,charged to a high potential by suitable means. The charge is deliveredthrough cable 3, support ele-ment 2, and tube 4 to head l. The paint isfed under a. slight pressure through tubes 4 and 5, fills up reservoir2l and is then forced through holes 22. to atomizing surface 20. At theopening of each hole 22 on surface a jet is formed due to the influenceof the high lield between the object or article lto be sprayed '18 whichis grounded and the highly charged atomizing head. The spray patternobtained is uniform without rotating the head l. rIltis eliminates theneed for rotating equipment with its attendant cost.

'4 The minimum separation between holes on the atomizing surface isgoverned by the base ydiameter of a liquid ,iet formed by the action ofthe electrostatic iield. The base diameter for a given set of conditions`can be determined by direct measurement of a single jet. In general,

the closer the holes are brought together, the greater the number ofjets that can be established per unit surface area. This tends tomaximize the rate of atomization and provides a more uniform spray.However, this has its limitations and if the holes are too closelyspaced, the iets tend to coalesce, increasing the size of the dropletssprayed therefrom with resulting waste and irregularity of pattern. Forniost conventional paints and lacquers, rit will be found desirable tomaintain the basediamcter of the jets within la range of about 0.015 toabout 0.04 incfh.

ln one preferred embodiment, the positioning of these holes is anon-center distance equal to the base diameter of the liquid jets to beformed at such holes. This embodiment `is illustra-ted in FIGURE 4. InFIGURE 4, individual gets 30 and 31 are shown each being fed by aditierent hole 22. Here the base of the jets barely touch at A. Inaddition to making use of essentially all of the atomizing surface, thisprovides the additional advantage of interposing a layer of dielectricmaterial, i.e., the coating liquid, between the charged atomizer headand the grounded article which serves |to further retard or prevent theformation of conditions admitting of corona discharge. This is ofsomewhat greater signiicance when the workpiece, i.e., the article to becoated, is made positive with respect to the atomizing apparatus.

An individual jet base diameter is a function of the intensity of theelectric field, the diameter of hole 22, the mass rate of fluid dow, theviscosity, conductivity and surface tension of the fluid. For a givenfluid and a certain diameter of holes 22, the jet base diameter variesinversely with the voltage applied to the spray head. The optimum valueof the Voltage on the atomizer head, therefore, would be in the range ofabout to 100 percent of the value of the voltage that could be appliedwithout objectionable corona discharge. All other factors being equal,the diameter of the base of a jet at a voltage within this range varieslittle and this distance is herein termed the minimum jet base diameterfor a specilied diameter ot' role 22 and a given fluid. Once theatomization has been initiated and the desired jet size attained, thevoltage may be lowered appreciably from the initiation voltage withoutany significant loss of eliiciency. For instance, with a surface uponwhich the optimum jet size is achieved at 150,000 volts, the voltage maybe reduced after initiation by as much as 20,000 to 30,000 volts withoutany noticeable change. This provides a relatively wide area of operationbelow the field intensity of the onset of corona discharge andsimplilies operational control.

The relationship between the pressure drop through holes 22 and thepressure differential between the top and bottom of the reservoir 21should be such that pressure drop through the holes is much greater thanthe pressure dilferential within the reservoir to assure that thereservoir will be completely lilled and the fluid will flow throughholes adjacent the top of the reservoir at substantially the same rateas through those :adjacent the bottom.

The pressure differential within the reservoir is due solely to thehydrostatic head within the reservoir. The pressure drop through theholes is expressed by the formula:

where DP is the pressure drop through the holes l is the length of thehole;

u is the viscosity of the liquid flowing through the hole; R is theradius of the hole; and

g is the volumetric rate of flow The pressure drop DP through the holesshould preferably be about l0 times the pressure differential from topto bottom of the reservoir to assure a uniform spray from the atomizerhead. This sets the upper limit upon the diameter of the holes for alluid of given viscosity.

ln a second preferred embodiment, the holes through which the fluid tobe sprayed is introduced to the atomizing surface are positionedslightly farther apart than in the embodiment hereinbefore discussed. lnthis embodiment, the base diameter of the huid jet is somewhat less thanthe distance between these feeder holes. Here, the holes are positionedso as to utilize the greatest possible amount of the atomization areawithout bringing the individual jets into contact with each other. Inthis embodiment, the distance between the feeder holes preferably wouldbe only slightly in excess of the base diameter of the liuid jets, i.e.,jet base diameters of about 90 to about 99.5 percent of the distancebetween holes. It has been found that this method of spacing and feedingminimizes or completely eliminates the tendency of closely spaced jetsto coalesce. This embodiment has proven to be particularly effectivewhen the atomizer head is a cylinder or the surface closely approximatesa cylindrical surface. Although the polarity of the article and atomizercan be reversed in each of the various embodiments of this process, itsurprisingly was found to be particularly desirable to operate with thearticle negative with respect to the atomizing surface when the base ofthe fluid jets are not permitted to overlap.

The invention will be more fully understood from the following7 examplewhich is herein set forth for purpose of illustration only and shouldnot be construed as a limitation upon the true scope of the invention,as set forth in the appended claims.

Example Electrostatic atomization, projection and deposition of variouscoating fluids and enamels was carried out in accordance with theinvention herein efore described employing an atomizer head ofcylindrical shape. The following equipment sizes and operatingconditions were employed:

The various coating materials employed in the aboverecited testsincluded the ollowing:

Paint resistivity, 1negohm/centimeter (l) Pigmented primers and enamels(o) epoxy and alkyd primers (b) alkyd melamine enamels, includingmetallics and non-metallics (c) acrylic paints, non-metallic (2) Clearailryd resins The solvents employed in the process of this invention arewell known in the art. The conductivity of some commercial paints orother coating iiuids may be such that it is difficult to use suchmaterial in an electrostatically propelled system without additions.Such paint should have included in their thinner Sonie polar solventsuch as methyl alcohol, acetone, etc. to raise their conductivity. Ifthe paint conductivity is satisfactory', it may be thinned byconventional non-polar solvents. For most applications, it is preferredto operate 6 with the paint at a viscosity between about 1.5 and about20 poise.

The terms coating liquid and coating fluid as employed herein includepaint, lacquer, ink, petroleum, the Various fractions and derivationsthereof, coal tar derivates and other coating materials that can beelectrostatically atomized, propelled and deposited upon a surface. Theterm coating tluid shall be understood to include both liquids andliquid coating materials which contain finely divided solid materialssuch as pigments, etc.

What is claimed is:

l. ln an apparatus for electrostatically spraying an object with acoating uid comrdsing a spray head, means for supplying said coating uidto said ay head, and means for maintaining an electrical potentialbetween the spray head and the object to be sprayed, the improvementwhich comprises a spray head having an interior surface and an exteriorcurved surface having a radius of curvature above about 0.25 inch andcommunicating with said interior surface through a plurality of smallopenings in said spray head.

2. ln an apparatus for electrostatically spraying an object with acoating fluid comprising a spray head, means for supplying said coatingfluid to said spray head, and means for maintaining an electricalpotential between the spray head and the object to be sprayed, theimprovement which comprises a spray head having an interior surface andan exterior curved surface having a radius of curvature above about 0.4inch and communicating with said interior surface through a plurality ofsmall openings in said spray head.

3. In an apparatus for electrostatically spraying an object with acoating fluid comprising a spray head having an opening therein throughwhich said coating fluid can be supplied for dispersal to said object,means for supplying said coating iuid to said opening, and means formaintaining an electrical potential between the spray head and theobject to be sprayed, the improvement which comprises a spray headhaving an interior surface and an exterior curved surface having aradius of curvature above about 0.25 inch and communicating wtih saidinterior surface throught a plurality of small openings in said sprayhead.

4. in an apparatus for electrostatically spraying an object with acoating fluid comprising a spray head having an opening therein throughwhich said coating fluid can be supplied for dispersal to said object,means for supplying said coating iiuid to said opening, and means formaintaining an electrical potential between the spray head and theobject to be sprayed, the improvement which comprises a spray headhaving an interior cavity and a curved exterior surface having a radiusof curvature above about 0.4 inch and communicating with said interiorcavity through a plurality of small openings in said spray headdistributed over a portion of said exterior surface subtending asubstantial central angle.

5. ln an apparatus for electrostatically spraying an object with acoating fluid comprising a spray head having an opening therein throughwhich said coating fluid can be supplied for dispersal to said object,means for supplying said coating fluid to said opening, and means formaintaining an electrical potential between the spray head and theobject to be sprayed sufficient to atomize, project, and deposit on saidobje-ct coating fluid supplied to said opening, the improvement whichcomprises a substantially cylindrical spray head having au interiorcavity and an exterior surface having a radius of curvature above about0.25 inch and communicating with said interior cavity through aplurality of small openings in said spray head distributed over aportion of said exterior surface subtending a substantial dihedralangle.

6. in an apparatus for electrostatically spraying an object with acoating fluid comprising a spray head having an opening therein throughwhich said coating fluid can 7 be supplied for dispcrsai to said object,means for supplying said coating fluid to said opening, and means formaintaining an electrical pecntial between 111e Spray head and theobject to be sprayed sucient o aomize, project, and deposit upon saidobject coating uid suppied to said opening, the improvement whichcompses a substantially spherical spray head having an interior cavityand an exterior Surface having a radius of curvature above about 0.25inch and communicating with said interior cavity hrough a pluraiy ofsmall openings in said spray head distributed over a porion of saidexeior surface subtending a substantial solid angle.

References @i1-ed in the fue of this patent UNITED STATES PATENT-S2,685,536 Starkey et ai. Aug. 3, 1954 3,017,115 Artman et ai Ian. 16,1962 FOREIGN PATENTS 166,458 Ausraa Ian. 5, 1756

1. IN AN APPARATUS FOR ELECTROSTATICALLY SPRAYING AN OBJECT WITH ACOATING FLUID COMPRISING A SPRAY HEAD, MEANS FOR SUPPLYING SAID COATINGFLUID TO SAID SPRAY HEAD, AND MEANS FOR MAINTAINING AN ELECTRICALPOTENTIAL BETWEEN THE SPRAY HEAD AND THE OBJECT TO BE SPRAYED, THEIMPROVEMENT WHICH COMPRISES A SPRAY HEAD HAVING AN INTERIOR SURFACE ANDAN EXTERIOR CURVED SURFACE HAVING A RADIUS OF CURVA-